The invention relates to a floating offshore construction, comprising a suspension gear for suspending a riser construction, the suspension gear being arranged to transmit downward force exerted by a riser construction on the offshore construction and comprising a hoisting gear or supporting the riser construction and a guide which, during use, extends adjacent the water surface.
Such an offshore construction is known from U.S. Pat. No. 3,858,401 and comprises a hoisting gear for supporting a riser construction built up of a plurality of gas receiving, buoyant chambers.
U.S. Pat. No. 3,017,934 discloses a floating offshore construction having a telescopic union for a riser construction which is not arranged to transmit downward force, comprising a guide which, during use, extends adjacent the water surface with a float that is connected to a riser construction. In use, the telescopic union can be extended from the float fixed to the upper segment of the riser construction by axially moving the guide into a tubular column suspended from the offshore construction.
The offshore construction according to U.S. Pat. No. 3,858,401 is used for the offshore exploitation and preparation for exploitation of submarine wells of natural resources in places where the seabottom lies relatively deep below the water surface. To be able to reach a well, the floating offshore construction, often a drilling ship or a semi-submersible, is positioned on the water surface above the well. Next, from the buoyant offshore construction, a riser pipe is lowered which is coupled to a stop it valve already provided on the seabottom, the riser forming a guarded conduit through which, for instance, during preparing the well for exploitation, drilling tools can be lowered and, during the exploitation, natural resources can be conveyed from the well to the offshore construction without these resources contacting water.
The riser construction is typically built up from riser segments which, are coupled during lowering and detached again during raising. Usually, this involves up or down displacement respectively of the riser construction over the length of one pipe segment by means of a hoisting gear forming part of the suspension gear. Due to the relatively great depth of the seabottom relative to the water surface, the offshore construction cannot, as in the case of a non-floating offshore construction, be supported by legs on the seabottom, but is buoyantly positioned above the well by means of ground anchors or dynamic positioning means. To enable the offshore construction to follow wave movements of the water surface relative to the riser construction, the suspension gear usually comprises a clamp coupling for receiving the riser construction which is connected to the offshore construction by means of telescopic cylinders and/or a tensioning system designed as cables running along pulleys, the suspension gear transmitting the downward force exerted by the lowered riser construction on the offshore construction. The offshore construction must have sufficient buoyancy to be able to compensate the downward force exerted by the riser.
Because of exhaustion of wells located in places where the bottom is relatively shallow, it is increasingly important to be also able to exploit and prepare for exploitation wells that are located in places where the seabottom is relatively deep. In particular, it is presently desired that it be possible to exploit wells located in places where the seabottom lies more than 1500 m below the water surface.
This entails the problem that the longer riser constructions required therefor exert a greater downward force on the offshore construction, so that the suspension gear should be of heavier design and the offshore construction should have a greater buoyancy. In practice, this leads to a considerable increase of the manufacturing costs and the operational costs of the offshore gear.
The object of the invention is to provide an offshore construction of the type mentioned in the preamble that does not have the above drawbacks. To that end, the offshore construction according to the invention comprises a suspension gear for suspending a riser construction, the suspension gear being arranged to transmit downward force exerted by a riser construction on the offshore construction and comprising a hoisting gear for supporting the riser construction and a guide which, during use, extends adjacent the water surface, characterized in that, the suspension gear further comprises a float arranged for axial movement in the guide, the float being provided with a coupling device for receiving the riser construction and in that the suspension gear further comprises a length-adjustable connecting device connecting the guide to the float. The effect achieved by the additional buoyancy of the float is that the downward force exerted on the floating offshore construction by the riser construction via the suspension gear can be reduced considerably, so that the suspension gear can be of a simpler design and the buoyancy of at the offshore construction can be smaller. Due to the axially movable arrangement of the float, it can move back and forth along the guide, when it is coupled to a riser construction, allowing the floating offshore construction to follow wave movements of the water surface. Further, by the guide, horizontal forces can be absorbed between the offshore construction and the riser construction, i.e. forces substantially in or parallel to the water surface, for instance due to current or wind. As a result, a vertically adjustable connection between the riser or the float and the offshore construction can be of a considerably simpler design, since it will now be substantially loaded in vertical direction or substantially transversely to the water surface.
In an advantageous embodiment, the guide comprises a conduit and the float comprises an elongated sleeve which is provided with a floating chamber and accommodated in the conduit for axial movement. The effect thus achieved, inter alia, is that transverse to the direction of movement, a proper power transmission is possible between the float and the offshore construction and that a reliable guidance can be realized in a simple manner. In particular, in this embodiment, the above-mentioned transmission of transverse forces can be realized highly effectively.
In another embodiment, the floating chamber is accommodated in the guide so as to be secured against axial rotation. The effect thus achieved is that the chance of accumulated torsion of the riser construction caused by the offshore construction following wave movements of the water surface, can be reduced.
In yet another embodiment, the offshore construction according to the invention is characterized in that the floating chamber is provided with controllable ballast means. The effect thus achieved is that an upward or downward movement of the riser construction relative to the offshore construction can be supported. This is in particular advantageous during upward or downward movement of the riser relative to the offshore construction during the assembly or disassembly of a riser construction built up from riser segments.
In a further embodiment, the offshore construction according to the invention is characterized in that the floating element has a central bore for guiding the riser therethrough. The effect thus achieved, inter alia, is that the riser construction, during lowering, can be lowered at a predetermined angle. Preferably, the central bore has sidewalls which, relative to the longitudinal axis of the guide, diverge in downward direction at an angle of 1-6xc2x0, preferably about 3xc2x0. To reduce the chance of damage to the riser construction by the sidewalls, the sidewalls may be provided with a protection, for instance a rubber lining.
In again another embodiment, the float is detachably connected to the guide. The effect thus achieved is that the offshore construction can be uncoupled from the float with the riser construction. In particular, the riser construction with the float can thus buoyantly remain behind above the well, while the offshore construction with the guide can be displaced as separate unit.
In again another embodiment, the offshore construction according to the invention comprises a guide which is height-adjustable to a position above the water surface. The effect thus achieved is that when no riser construction is present, the guide can be adjusted to a position above the water surface, so that during travel, a more favorable flow resistance can be obtained. The invention also relates to a float.